The present invention relates to methods for the separation of high purity silicon crystal from hyper eutectic binary or ternary Si containing alloys or from refined silicon melts.
The development of new techniques and products for the low-cost utilization of non-polluting energy sources has attracted considerable interest during the latest decades. Solar energy is one of the most interesting energy sources since it does not cause any to pollution and since it is abundantly available. Solar energy is converted into electricity by means of the photovoltaic effect, whereby the sunlight is absorbed by so called solar cells. The aim of the present invention is to provide low-cost silicon materials for use in solar cells.
Traditionally solar grade silicon has been produced by the Siemens process. The key feature of this process is chemical vapour deposition (CVD). The Siemens process is an energy consuming and high-cost process. The material obtained by the Siemens process is of a purity (>11N) suitable for electronic applications such as semiconductors. Silicon for Solar cell materials may have a lower purity (>5N) and still be suitable for use in the photovoltaic process, wherein N designates the number of 9-digits in the purity of the material.
In recent years a direct process has been used, making use of quartz and carbon, the so called Elkem or Fesil process. The product is molten silicon and is commonly known as UMG-Si (upgraded metallurgical grade silicon) or SoG-Si (Solar grade silicon). The process operates at high temperature (>1400° C.) and is thus also a process consuming considerable energy and materials choice is a big challenge at those temperatures.
U.S. Pat. No. 4,124,410 describes a multigrained, epitaxial n-on-p-on-p substrate planar layer diode comprising p- and n-type epitaxial layers grown on a passive p-type, multigrained, silicon substrate where said layers duplicate the underlaying substrate grain secondary structure. The silicon substrate is prepared by removing silicon platelets from a solution of metallurgical grade silicon in a liquid metal solvent, melting of said partially purified silicon platelets in contact with acid silica slag in a melting zone and pulling refined metallurgical silicon boules from a melt of thus refined silicon on a rotating silicon seed rod. The substrate is produced at a substantially higher impurity level than in conventional high purity semiconductor grade silicon, while the resulting low-cost diode have solar cell properties.
WO 2007/112592 A1 describes a method for purifying silicon. In said method a first molten liquid is formed from silicon and a solvent metal, this first molten liquid is then contacted with a first gas to provide dross and a second molten liquid, whereupon the dross and the second molten liquid is separated. The second molten liquid is then cooled to form silicon crystals and a mother liquid, the crystals are then separated from the mother liquid.
All the known methods for the preparation of silicon suitable for use as solar cells are thus methods consuming considerable costs and energy.
It is an object of the invention to provide refined metallurgical material useful for solar is cell applications or readily convertible to such silicon material.
The object of the present invention is thus to provide an alternative process for the preparation of solar grade silicon which is less energy consuming and which allows greater flexibility with regard to starting materials.